Method of bleaching pulp



June 4, 1940. F G, RAWLlNG 2,203,205

METHOD 0F BLEACHINGPULP Filed Oct. 8, 1937 Patented June 4, 1940 PATENT OFFICE METHOD OF BLEACHING PULP Francis G. Rawling, Piedmont, W. Va., assigner to West Virginia Pulp & Paper Company, New York, N. Y., a corporation of Delaware Application October 8, 1937, Serial No. 167,902

1 Claim.

My present invention relates to bleaching of pulp especially of the types known as suliite, soda, kraft, monosulte and the semi-cooked pulps.

In bleaching pulps of these types it is custom- 5 ary to treat the pulp with chlorine or other bleaching agents up to around 70% of the total chemicals required to bleach to a white color, and then wash the pulp. The washed pulp is then treated with an alkali such as caustic soda, lime,

sodium carbonate, etc., at room or elevated temperatures to remove lignin material and bleach residues. The pulp is again washedl and submitted to further treatments with hypochlorites either under neutral or alkaline conditions, for

periods of time up to six hours, with a washing 'step between each treatment, until the pulp has attained a suicient degree of whiteness.

The number of washing steps required in the treatment above outlined adds substantially to the cost of the treatment. According to my present invention I eliminate a number of these washing steps by changing the character of the' bleaching solution simply by altering the pH of the solution through a substantial range, as for example, by carrying out a hypochlorite bleach at a pH of not less than 8, and then by adding acid, converting the bleach into hypochlorous acid, HClO, at a pH of approximately 7, and then upon further addition of acid to elemental chlorine at a pH of 5, time being allowed for reaction on the pulp at these hydrogen ion concentrations. The pH is then changed to the alkaline side by addition of sodium hydroxide, for example, thereby reversing the cycle and ending up with sodium hypochlorite. If necessary, this pH cycle may lbe repeaied. The material is then washed and is ready for the beaters.

The invention is illustrated in the attached flowsheet which may be read with the following detailed description of a method of carrying out my invention according to a preferred embodiment thereof.

A suitable lquantity of kraft pulp is treated with about 5 to 8% of its weight with calcium hypochlorite solution at a pulp consistency of around 5%, Without washing, enough chlorine is added so that a slight excess remains .at the .end of thirty minutes treatment. The pulp is then washed, treated with caustic soda until the pH of the pulp remains about 10 to 11. The pulp is then washed and is then submitted to alternate bleaching treatments as follows:

A hypochlorite solution about 3% on the weight of the pulp is added to the pulp suspension, is mixed for a period of about ten minutes, where- -upon sulfuric acid is added until the pH of the pulp reaches a nal valueof around 5. At this stage free chlorine will be present in the pulp in accordance with the following equation:

2Naoc1+Hzsoi-NazsoH-mocl 5 with further addition of acid,

Hoc1+Nac1+H2so4 NaHsol+H2o+C12 If calcium hypochlorite is used the reaction is 10 similar.

After maintaining the solution acid for approximately 15 minutes, alkali is added until the, pH becomes preferably between 9 and 10 and the solution held at this stage for approximately an- Il other 15 minutes. 'I'he pulp is then washed and, if necessary, again treated with a small quantity of hypochlorite and/or chlorine.

'I'he treatment of the pulp may be made on batches of pulp or in a continuous bleaching sys- 20 tem by the addition of the acid and alkali at proper points in the continuous apparatus. Enough hypochlorite should be added prior to the acidication so as to aiord suilcient chlorine to carry through the cycleY of pH change. If in- 25 suflicient hypochlorite is present, more should be added to the pulp. In place of hypochlorites, chlorine may be added to the pulp, since it will form hypochlorites when the pulp is alkaline.

If desired the bleaching action can be further 30 intensified by blowing air into the pulp, since the dissolved oxygen removes coloring material from the pulp in the presence of alkali, as described and claimed in my copending application, Serial No. 72,059, iiled April 1, 1936, Patent Num- 35 ber 2,147,618 patented February 14, 1939.

An alternative method of bleaching following the initial treatment of the pulp with hypochlorite then with chlorine, followed by Washing, an alkaline treatment and a secondtwash is the reo placement of the hypochlorite -added to the pulp after the second wash, with chlorine. This chlorine should be added to the pulp in suicient quantity to carry through the cycles of change of pH values. Whenever the pH-value of the pulp 45 is raised to 9 or 10, or so by the addition of alkali,

' hypochlorites are formed and the cycle of change of pH value of the pulp from 10 or thereabouts to 5 and back to 10 can be made by the alternate addition of acid and alkali until the desired color 50 or whiteness is obtained, after which the pulp ls washed.

Various other changes maybe made in the specic example Without departing from the spirit of my invention or the scope of the appended claim. 55

I claim:

In the bleaching of pulp in which the pulp is subjected to a preliminary intensive treatment with a chlorine bleaching agent followed by alkali treatment and removal of the soluble bleach residues by Washing, the method of carrying out an after-bleach comprising subjecting the partially bleached pulp to alternate treatments with chlorine and alkaline hypochlorite, without Washing between the treatments, by rst treating the pulp by adding one of said compounds (chlorine or hypochlorite) to the pulp in an amount in excess of that required to complete the bleaching in one stage, and thereafter appropriately varying the pH of the treated pulp by adding alkali in the case of chlorine treatment and acid in the case of the hypochlorlte treatment to produce a pH over a range of from an excess of 9 to not substantially more than 5 to form the other compound in situ from said excess of the rst compound, and alternately ycontinuing such treatments each for a period on the order of from 10-15 minutes, whereby the pulp in the afterbleach is subjected to a series of comparatively rapid continuously oxidizing bleaching treatments each of which is alternately acid or alkaline, as the case may be.

FRANCIS G. RAWIING. 

